In high-capacity, long-haul fiber data transmission, digital signal processing (DSP) after signal detection can greatly improve transmission performance by providing compensation against fiber impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). Together with polarization diversity coherent detection, DSP also grants polarization division multiplexing (PDM) to double the transmitted data rate. To implement DSP for post detection processing, analog-to-digital converters (ADCs) at high sampling rate are needed in the coherent receiver design. In reality, the signal bandwidth that can be supported by electronic ADC is much lower than the E-O modulator bandwidth at the transmitter. Currently, the state of the art is about 20 GHz for electronic ADC bandwidth, while 40 GHz bandwidth E-O modulators are already commercially available. To fully utilize the potential of post-detection DSP at high data rates, a new ADC technology is required to fill the bandwidth gap.
Parallel processing of incoming high BW signals is often used to achieve high ADC sampling rates. Electronic time-interleaved ADCs have synchronized track-and-hold circuitry, which has to cover the entire input signal BW (state of the art ˜20 GHz), on each parallel sampling path. Parallel processing in frequency domain, which can reduce the BW requirement of each sampling path, has been proposed to reach higher sampling rates. We proposed recently a photonic filter bank (PFB) structure using orthogonal filter design to allow digital perfect reconstruction in theory. In order to sample the high frequency tributary of the incoming signal, filter bank methods need RF electronic frequency down converters. The need for rf down conversion not only increases system complexity and cost (a typical phase/polarization diversity receiver requires a total of four converters), but it may also degrade system performance because of the difficulty of analog wideband processing in electronic domain. In terms of filter design, it is desirable to have sharp filter roll-offs because of the limited ADC BW in each parallel sampling paths. In PFB structure, sharp roll-offs are challenging to implement using orthogonal filter design because many optical taps are required.
Accordingly, there is need for a method to generate over 100 Gbit/s OFDM signals with the limited bandwidth for A/D and D/A converter tolerance.